Has Voyager 1 left the heliosphere? The question is a reminder that the Voyagers are our first interstellar probes; they’ll still be returning data when they move into the interstellar medium. The heliosphere is a kind of bubble created by the solar wind from the Sun, that stream of high-speed charged particles constantly blowing into space at roughly 400 kilometers per second. Observing how Voyager 1 makes the transition across the boundary of the heliosphere will provide our first in situ study of interstellar space.

Some scientists believe that at roughly 90 AU from the Sun, Voyager 1 has already pushed up against the ‘termination shock,’ that region where the speed of the solar wind drops to subsonic levels. Now new data studied by French and Finnish researchers indicate that the shape of the heliosphere may be distorted, further complicating the question of just where the true interstellar medium begins.

Rosine Lallement and colleagues used data collected by the Solar and Heliospheric Observatory (SOHO), studying the direction of helium and hydrogen particles from the area where the heliosphere and interstellar space begin to interact. Their findings suggest that Voyager 1 remains within an elongated region well within the heliosphere. We’ll be learning much more about the heliosphere’s boundaries as Voyager continues its extended mission, returning data perhaps as late as 2020.

The paper is R. Lallement, E. Quemerais et al., “Deflection of the Interstellar Neutral Hydrogen Flow Across the Heliospheric Interface,” Science 307, pp. 1447-1449 (4 March 2005).